Use a tag to determine if a slab should be supported

[BlayTheNinth]

Nowadays, slab implementations are uniform enough that the increased complexity of the json compat files should no longer be necessary. At this point, every compat file uses just simple slab listings since the migration to block states and the flattening made most mods use the vanilla slab class as a base. We can merge SmarterSlabConverter and SmartSlabConverter, get rid of SimpleSlabConverter, and use a kleeslabs:slabs tag to identify slab blocks instead.